1. Field of the Invention
The present invention relates to an arc tube in which electrodes including filament coils are sealed at ends of an arc tube body, and a low-pressure mercury lamp including the arc tube.
2. Related Art
With the advent of the energy-saving era, research is being performed into low-pressure mercury lamps such as fluorescent lamps. In particular, increasing attention has been given to compact self-ballasted fluorescent lamps as alternative light sources to incandescent lamps. As an example, a compact self-ballasted fluorescent lamp includes a 3U-type arc tube in which three glass tubes bent in the shape of U are connected to form an arc tube body (e.g. Japanese Patent Application Publication H09-231825).
One long discharge space is formed in this 3U-type arc tube. Electrodes are sealed at both ends of this discharge space (i.e. both ends of the arc tube body). Each of the electrodes includes a filament coil and a pair of lead wires supporting both ends of the filament coil.
The filament coil is a multiple-coiled filament which is formed, for example, by double-coiling a wire and then further coiling the double-coiled wire a plurality of turns around a predetermined mandrel.
Each electrode is sealed at the corresponding end of the arc tube body in the following manner. The electrode is inserted into the end of the arc tube body from the filament coil side, until the filament coil reaches a predetermined position in the arc tube body. In this state, the end of the arc tube body is heated and pinched (by application of pressure).
In recent years, there has been an increasing demand for smaller low-pressure mercury lamps. This being so, the need for compact self-ballasted fluorescent lamps which are equal in size to or even smaller than incandescent lamps is growing too. This creates a recent trend toward smaller arc tubes, by reducing the diameter of the glass tube which constitutes the arc tube body to thereby downsize the art tube body.
However, such downsizing of arc tubes causes the following problems. Suppose a glass tube having an inside diameter of 9 mm or less is used to form an arc tube body. A conventional electrode cannot be inserted into such an arc tube body, since a length of a filament coil of the electrode along a coil axis direction is greater than the inside diameter of the glass tube.
If the filament coil is wound with a smaller pitch in the last coiling stage of its multiple coiling stages, the length of the filament coil along the coil axis direction is reduced, with it being possible to seal the electrode at the end of the arc tube body. In this case, however, adjacent winding turns of the filament coil become closer to each other. This being so, if the filament coil touches an inside surface of the arc tube body and becomes deformed when the electrode is being inserted into the arc tube body or if the electrode vibrates when the electrode is being sealed at the end of the arc tube body or when the arc tube is being transported as a completed product, adjacent winding turns may touch each other (this is called a coil touch).
When a coil touch occurs, the filament coil fails to reach a desired temperature when energized. This causes an electron emissive material on the filament coil to remain without being decomposed, which results in a loss of life or a lighting failure of the lamp.